1. Field of the Invention
The invention relates to an apparatus and a method for measuring a wavelength-dependent optical characteristic of an objective, a lens or any other optical system. In particular, the wavelength-dependent optical characteristic can be the spectral modulation transfer function (MTF), the spectral transmission coefficient T(λ), the longitudinal chromatic aberration or the transverse chromatic aberration.
2. Description of the Prior Art
The prior art has disclosed apparatuses for measuring the modulation transfer function of an optical system, in which a light pattern, e.g. a line, a lined cross or a point, is generated in a focal plane of the optical system. To this end, it is possible, for example, to arrange a stop in the focal plane of the optical system, which stop is illuminated by a light source and has a corresponding aperture. The optical system images the aperture to infinity.
A camera with a lens and a light sensor, which is arranged in a focal plane of the lens, is arranged on the opposite side of the optical system. The object images the image from infinity on the light sensor. The modulation transfer function can be derived from the image on the light sensor. Moreover, it is known to use a stop with a plurality of apertures and arrange a plurality of cameras on the opposite side of the optical system with such a distribution that each camera acquires, with a light sensor thereof, the image from exactly one aperture. In this way, it is possible to measure the modulation transfer function at a plurality of field positions independently of one another.
Moreover, apparatuses are known, in which the stop with the pattern of apertures is imaged to infinity by a collimator such that the optical system to be measured is illuminated in a collimated manner. Then, the lens of the camera images an intermediate image of the stop, which is generated by the optical system, on the light sensor.
In order to determine the modulation transfer function for different wavelengths so as to derive the spectral modulation transfer function MTF(λ) therefrom, use has previously been made of light sources, the emission spectra of which can be changed in a discrete manner. Such a light source can e.g. have a plurality of LEDs with different emission wavelengths, which are successively introduced into the light path. As an alternative thereto, use is made of white-light sources, the light of which is filtered by color filters which are interchangeably arranged in the light path. The modulation transfer function is then measured separately in succession for each wavelength.
However, such a measurement is too slow if optical systems are intended to be measured in great numbers in applications close to production.
For the purposes of measuring the spectral transmission coefficient T(λ), measurement light is directed onto a slit after passing through the optical system, said slit being imaged on a light sensor via a prism, a grating or any other dispersive optical element.